Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or multiple channels (e.g., one or more of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel. For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel, or channels. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver receives RF signals, demodulates the RF carrier frequency from the RF signals via one or more intermediate frequency stages to produce baseband signals, and demodulates the baseband signals in accordance with a particular wireless communication standard to recapture the transmitted data. The transmitter converts data into RF signals by modulating the data in accordance with the particular wireless communication standard to produce baseband signals and mixes the baseband signals with an RF carrier in one or more intermediate frequency stages to produce RF signals.
In both the receiver and transmitter sections of a wireless communication device, bandpass filtering and low pass filtering are critical for proper operation. In radio frequency integrated circuits, such filtering is typically achieved using a standard lumped element filter design, which has at least three primary issues, especially for CMOS technology. In particular, the three issues include passive design synthesis, passive design loss, and stability. The passive design synthesis issue becomes significant when a passive filter is designed for narrow band operation in the gigahertz range. For instance, a standard Chebychev design produces a minimal 2 pole bandpass filter, which has large series inductors (e.g., 50-60 nano Henries) and very small shunt inductors (0.1-0.2 nano Henries). For integrated circuit design, the series inductors, if they are to have a substantial Q factor, are very large, i.e., consume a significant amount of integrated circuit real estate, while the shunt inductors are very small. As is known, very small inductance values are sensitive to process variations, which can provide percentage variations of the inductance value in the order of 100% or more, which completely destroys the desired filtering properties.
In addition, the Chebychev design procedure results in a similar problem for capacitors. In particular, the series capacitors are very small (e.g., 0.5-0.1 pico Farads) and can be completely over shadowed by the parasitic capacitance of the inductors, adjacent signal lines, et cetera making the filter not operate as desired. Conversely, the shunt capacitance is very large (e.g., 40 pico Farads) and occupies a significant amount of integrated circuit real estate. As such, the standard design procedure yields unacceptable filters for integrated circuits including radio frequency integrated circuits.
Passive design loss results from energy losses due to the metal and substrate of at least some of the elements in a filter. In some instances, the loss may be as much as 20 dB. To remedy the passive loss, active elements should be introduced into the passive filter design. As with any filter, if the filter includes an active element it must be created in such a way to be unconditionally stable. As is known, if a filter becomes unstable, it acts as an oscillator injecting unwanted oscillations into the system.
Therefore, a need exists for on-chip filters that include realizable passive components, have reduced passive losses, and are unconditionally stable.